Unsaturated epoxy ester resins, or epoxy acrylates, are widely used as various coating materials and structural materials because of their outstanding chemical resistance, water resistance and mechanical properties. Such unsaturated epoxy ester resins are usually produced by esterifying an epoxy compound with a polymerizable unsaturated carboxylic acid in the presence of a catalyst and are often used in combination with a photopolymerization initiator as a photosensitive resin. Improved properties such as adhesiveness, thermal resistance, photocurability and, at the same time, heat-curability can be imparted to the resin by further incorporating an epoxy resin and its curing catalyst therein to prepare a photosensitive composition.
However, when this photosensitive composition is employed as an image-forming material such as a resist material for preparing a printed circuit board, an organic solvent is usually necessary for the development step, which results in an environmental problem of handling the organic solvent.
Recently, in order to avoid this problem, a carboxylated unsaturated epoxy ester resin which is soluble in an aqueous alkaline solution due to the presence of the carboxyl group in its molecule has been prepared by reacting the hydroxyl group in the unsaturated epoxy ester resin as described above and an acid anhydride to introduce a carboxyl group therein. Further, when an epoxy compound and its curing catalyst are added to this carboxylated unsaturated epoxy ester resin, not only the development with an aqueous alkaline solution is possible, but also, in addition to water-resistance, adhesiveness and thermal resistance can be advantageously imparted by heat treatment after the development.
However, the widely used photosensitive resin compositions, comprising an unsaturated epoxy ester and an epoxy resin plus an epoxy curing agent such as a polycarboxylic acid, or comprising a carboxylated unsaturated epoxy ester resin plus an epoxy resin, contain a residual catalyst. This catalyst is used for the synthesis of the unsaturated epoxy ester by reaction of an epoxy compound and an unsaturated monocarboxylic acid. Even without particular addition of a catalyst, a curing reaction between the epoxy group and its curing agent will proceed slowly and gradually. This formulation therefore has a flaw of having little storage stability. For this reason, these photosensitive resin compositions are being offered for sale as a two component type composition where the epoxy resin component and polyvalent carboxylic acid component are supplied separately.
To solve this problem, the amine compound which could act as a curing catalyst is removed by washing the unsaturated epoxy ester resin after its synthesis with an aqueous acidic solution. However, this process which requires a longer period of time and a drying step is disadvantageously complicated and high in cost.